The invention relates to a fuel pump for use in supplying fuel to a compression ignition internal combustion engine.
A known high pressure fuel pump for use in supplying fuel to a compression ignition internal combustion engine comprises a pumping plunger reciprocable within a plunger bore under the influence of a cam drive arrangement. It is usual to provide a low pressure pump, for example in the form of a vane pump, to permit charging of a pump chamber of the high pressure pump within the time available. However, the provision of such a separate low pressure pump results in the fuel system being relatively complex, bulky and expensive.
It is an object of the invention to provide a fuel pump in which the provision of an auxiliary low pressure pump can be avoided.
According to the present invention there is provided a fuel pump comprising a pumping plunger reciprocable within a bore formed in a pump housing under the influence of a cam drive arrangement, the cam drive arrangement including a reciprocable tappet member, a surface associated with the tappet member defining, in part, an auxiliary pumping chamber, and a flow path interconnecting the auxiliary pumping chamber and the bore such that fuel displaced from the auxiliary pumping chamber, in use, can be supplied through the flow path to the bore.
By using the motion of the tappet member to pump fuel at relatively low pressure to the bore, the provision of a separate, auxiliary low pressure pump can be avoided thereby reducing the complexity, weight and cost of a fuel system including the pump.
The auxiliary pumping chamber is conveniently defined between the tappet member and the pump housing. The tappet member is preferably moveable to a position in which the tappet member is spaced from the pump housing, permitting fuel to flow to the auxiliary pumping chamber. An outlet valve is conveniently provided in the flow path to prevent the return of fuel to the auxiliary pumping chamber. A metering valve, for example a piezoelectrically controlled metering valve, may be provided in the flow path.
A pressure regulator may be provided to control the fuel pressure within the flow path. The pressure regulator conveniently controls the fuel pressure by controlling the rate at which fuel is able to escape from the flow path.
Preferably, the tappet member is reciprocable within an additional bore provided in a pump body part. The tappet member is preferably provided with means for substantially preventing relative angular movement of the tappet member with respect to the pump body part. For example, the pump body part may carry a pin which rides within a slot provided in the tappet member.